Problem $6.$ Overall System Gain for Feedback Control Systems

A block diagram is shown below for a feedback control system.

What is the overall system gain?

$($A$)\frac{r(t)}{f(t)}=\frac{\text{A}\text{B}\text{D}}{1+\text{A}\text{B}\text{C}-BD}$

$($B$)\frac{r(t)}{f(t)}=\frac{\text{A}\text{B}\text{D}}{1-\text{A}\text{B}\text{C}+BD}$

$($C$)\frac{r(t)}{f(t)}=\frac{\text{A}\text{B}\text{C}}{1+\text{A}\text{B}\text{D}-BC}$

$($D$)\frac{r(t)}{f(t)}=\frac{\text{A}\text{B}\text{C}}{1-\text{A}\text{B}\text{D}+BC}$